The invention relates to an optoelectronic sensor comprising a light source for the transmission of a scanning beam, a light deflecting unit, which is formed for the periodic deflection of the scanning beam within an angular transmission range, and a receiving device for the detection of light reflected and/or back-scattered from an object to be sensed.
With such sensors objects can be sensed and markings, for example bar codes applied to the objects, can be recognized. In order to be able to evaluate sensed object information a synchronization of the electronic detection and/or evaluation circuit connected to the receiving device with the periodic scanning movement of the scanning beam is required. This signifies that the start of the scanning period, i.e. the point in time at which an active reading process starts and from which point on reflected or back-scattered light is evaluated, must be communicated to the electronic system coupled to the receiving device.
It is an object of the invention to provide an optoelectronic sensor of the initially named kind which permits the determination of the start of the scanning period.
This object is attained by providing a deflection unit which has a light entry surface arranged at least regionally within the angular transmission range, which is formed for the deflection of light incident through the light entry surface onto a photo-detector, and which can be coupled to a synchronization circuit to produce a synchronization pulse to determine the start of a sensing period.
Thus, in accordance with the invention, the determination of the start of a sensing period, i.e. the synchronization, takes place by an optoelectronic route in a simple and almost delay-free manner.
Through the arrangement of the deflection unit within the angular transmission range, no additional space is required for the deflection unit within the sensor. Neither the extent of the angular transmission range nor that of the receiving range for reflected or back-scattered light are notably restricted by the deflection unit.
In accordance with a preferred embodiment of the invention, the deflection unit includes a plate of light-permeable material, preferably formed as an irregular polygon, and in particular as a four-cornered plate, in particular of plastic, the small sides of which are formed as light entry, light exit, and reflection surfaces.
In this way the plate works as a light conductor, in which light incident through a narrow side serving as an entry surface is deflected by reflection or total reflection at the other narrow sides in the direction of an outlet surface. Since such a light conducting plate simultaneously provides light entry, light exit and light reflection surfaces and also serves as a propagation medium for the light, the number of components required for the reflection of the light is minimized by forming the deflection unit as such a light conducting plate.
Furthermore, a light conducting unit of this kind can be made simply and at favorable cost.
In accordance with a preferred embodiment of the invention, the plane defined by the light entry surface of the deflection unit extends at least approximately perpendicular to the direction into which the scanning beam can be deflected before the start of or after the end of a scanning period by the light deflection unit.
In this way the comparatively large side surfaces of the preferably flat, light conducting plate do not face in the direction into which the scanning beam is deflected before the start of or after the end of the scanning period, but rather only their narrow sides do so. As a consequence, only a minimal part of the angular transmission range, which lies outside of an active angular reading range, is influenced by the deflection unit.
The light deflection unit is preferably formed as a polygonal mirror wheel, which is rotationally driven in operation and provided with reflecting mirror facets. The light beam transmitted by the light source produces a light bead on the facets of the mirror wheel, with a finite extent of, for example, a few square millimeters. As a result of this extent, only a part of the light beam, namely only the part of the light beam falling on the relevant facet, is deflected into the desired direction at the start of the scanning period by the front region of the relevant facet in the direction of rotation. This part of the light beam, i.e. the first scanning beam, is in many cases too low in intensity and thus unsuitable or only conditionally suitable for scanning purposes. The same applies to the last scanning beam, which is reflected by a rear region of the respective facet in the direction of rotation.
Accordingly, in accordance with a further preferred embodiment, the deflection unit is arranged in a region into which the scanning beam can be deflected by the light deflection unit before the start of or after the end of a scanning period, in particular by a front or rear marginal region in the direction of rotation of a reflecting facet of a light deflection unit formed as a rotatable polygonal mirror wheel. This makes it possible to use a scanning beam for synchronization purposes, which is not capable of being used for scanning or only conditionally capable of being used for scanning. Accordingly, practically all the light beams fully reflected from the respective facet are available as scanning beams suitable for the actual scanning, so that practically the entire angular transmission range can be used for the scanning, i.e. as an active reading range.
In accordance with further preferred embodiments of the invention, a light exit surface of the deflection unit is matched to a light sensitive surface of a photo-detector, and/or has at least substantially the same size as the light sensitive surface of the photo-detector, and/or the projection of the light exit surface of the deflection unit onto the light sensitive surface of a photo-detector in a direction extending approximately perpendicular to a plane defined by the light exit surface lies fully within the light sensitive surface of the photo-detector.
In this way a situation is achieved in which at least a large part of the light incident on the light entry surface passes to the photo-detector after single or multiple reflection or total reflection at the narrow sides of the plate.
In accordance with a further preferred embodiment of the invention the plate is connected to a base element, which preferably extends perpendicular to the plate and which is provided with positioning spigots projecting preferably perpendicularly for engagement with cutouts formed in a sensor circuit board. In this way it is possible to fix the deflection unit without complicated adjustment work at the desired location and in the desired position on a pre-manufactured circuit board provided with positioning bores. In particular a sensor circuit board, which is in any event present, serves for this purpose, so thatxe2x80x94with the exception of the deflection unitxe2x80x94no further components are required.
Further advantageous embodiments of the invention are set forth in the following description and in the drawing.